1. Field of the Invention
This invention relates to a display apparatus, a driving method for a display apparatus and an electronic apparatus, and more particularly to a display apparatus of the flat type or flat panel type wherein a plurality of pixels including electro-optical devices are disposed in rows and columns, that is, in a matrix, and a driving method for the display apparatus and an electronic apparatus including the display apparatus.
2. Description of the Related Art
In recent years, in the field of display apparatus for displaying an image, flat type display apparatus wherein pixels or pixel circuits including light emitting devices are disposed in a matrix have been popularized rapidly. As a flat type display apparatus, a display apparatus which uses an electro-optical device of the current driven type whose emission light luminance varies in response to the value of current flowing through the device, for example, an organic EL (Electro Luminescence) display apparatus which uses an organic EL device which utilizes a phenomenon that an organic thin film emits light when an electric field is applied thereto, has been developed and commercialized.
The organic EL display apparatus has the following characteristics. In particular, it exhibits low power consumption because the organic EL device can be driven by an application voltage lower than 10 V. Further, since the organic EL device is a selfluminous device, the organic display apparatus displays an image of high visual observability in comparison with a liquid crystal display apparatus wherein the intensity of light from a light source or backlight is controlled by the liquid crystal cell for each pixel including a liquid crystal cell. Besides, since the organic EL display apparatus does not require an illumination member such as a backlight which is essentially required by a liquid crystal display apparatus, it is easy to reduce the weight and the thickness thereof. Further, since the response speed of the organic EL device is approximately several μsec and very high, an afterimage upon dynamic image display does not appear.
The organic EL display apparatus can adopt a simple or passive matrix method and an active matrix method as a driving method therefor similarly as in the liquid crystal display apparatus. However, although the display apparatus of the passive matrix type is simple in structure, it has such a problem that, since the light emission period of the electro-optical devices decreases as the number of scanning lines or the number of pixels increases, it is difficult to implement a display apparatus of a large size and of high definition.
Therefore, in recent years, a display apparatus of the active matrix type has been and is being developed energetically wherein the current flowing to an electro-optical device is controlled by an active device provided in the same pixel circuit as the electro-optical device such as, for example, an insulating gate type field effect transistor, usually a thin film transistor (TFT). A display apparatus of the active matrix type can be easily formed as a display apparatus of a large size and high definition because the electro-optical device continues to emit light for a period of one frame.
Incidentally, it is generally known that the I-V characteristic, that is, the current-voltage characteristic, of an organic EL device deteriorates as time passes, that is, exhibits time degradation. In a pixel circuit which uses a TFT of the N-channel type as a transistor for current-driving an organic EL device (such a transistor is hereinafter referred to as driving transistor), since the organic EL device is connected to the source side of the driving transistor, if the I-V characteristic of the organic EL device suffers from time degradation, then the gate-source voltage Vgs of the driving transistor varies. As a result, also the emission light luminance of the organic EL device varies.
This is described more particularly. The source potential of the driving transistor depends upon the working point of the driving transistor and the organic EL device. Then, if the I-V characteristic of the organic EL device deteriorates, then since the working point of the driving transistor and the organic EL device varies, even if the same voltage is applied to the gate of the driving transistor, the source potential of the driving transistor varies. Consequently, the gate-source voltage Vgs of the driving transistor varies, and the value of current flowing through the driving transistor varies. As a result, also the value of current flowing through the organic EL device varies, and this varies the emission light luminance of the organic EL device.
Meanwhile, a pixel circuit which uses a polycrystalline silicon TFT suffers not only from time degradation of the I-V characteristic of the organic EL device but also from secular change of the threshold voltage Vth of the driving transistor or the mobility of a semiconductor thin film which composes a channel of the driving transistor (such mobility is hereinafter referred to as mobility μ of the driving transistor). Further, with the pixel circuit, the threshold voltage Vth or the mobility μ differs for each pixel from a dispersion in the fabrication process. In other words, each transistor has a dispersion in characteristics.
Where the threshold voltage Vth or the mobility μ of the driving transistor differs for each pixel, also the value of current flowing to the driving current disperses for each pixel. Therefore, even if the same voltage is applied to the gate of the driving transistors of the pixels, a dispersion in the emission light luminance of the organic EL device appears between the pixels. As a result, uniformity of the screen image is damaged.
Therefore, in order to keep the emission light luminance of the organic EL device fixed without being influenced, even if the I-V characteristic of the organic EL device suffers from time degradation or the threshold voltage Vth or the mobility μ of the driving transistor suffers from secular change, by such time degradation or secular change, the following configuration is adopted. In particular, each pixel circuit is provided with a compensation function for the characteristic variation of the organic EL device or a correction function for correction against the variation of the threshold voltage of the driving transistor (such correction is hereinafter referred to as threshold value correction) or for correction against the variation of the mobility μ of the driving transistor (such correction is hereinafter referred to as mobility correction). The configuration just described is disclosed, for example, in Japanese Patent Laid-Open No. 2006-133542.
By providing each pixel circuit with a compensation function for the characteristic variation of the organic EL device and correction functions against the threshold voltage Vth and the mobility μ of the driving transistor in this manner, even if the I-V characteristic of the organic EL device suffers from time degradation of the threshold voltage Vth or the mobility μ of the driving transistor suffers from secular change, the emission light luminance of the organic EL device can be kept fixed without being influenced by such time degradation or secular change as described above.